1. Field of the Invention
The present invention is concerned with improved modified starch-polyvalent metal derivatives and methods of preparation thereof, wherein the starches are preferably preswelled or expanded and both chemically cross-linked and substituted with hydrophobic moieties prior to substitution with polyvalent metals. The resultant starch derivatives are free-flowing in a dry state and exhibit extremely rapid and easy hydration in hot or cold water. The preferred starches are prepared by initially preswelling and cross-linking native starch, followed by subjecting the preswelled/cross-linked starch to a substitution reaction using a hydrophobic agent such as an acid or acid anhydride; thereafter, the starch is reacted with a metal salt resulting in metal substitution through bridging of hydrophobic moieties.
2. Description of the Prior Art
The preparation of starches including hydrophobic substituent groups is well known. U.S. Pat. No. 2,661,349 describes the preparation of such products by a reaction of starch in an aqueous alkaline slurry and in the presence of a hydrophobic agent. The '349 patent also describes other preparative methods using an organic suspension or discussion.
The esterification of starch with hydrophobic substituents such as octenyl succinic anhydride is described by Trubiano (Modified Starches: Properties and Uses, p. 131–147 (1986)) and Rutenburg et al. (Starch Chemistry and Technology, 2nd ed. p. 341–343 (1984)). Such prior art substituted starch products tend to swell excessively and fuse together upon discussion in water and heating above the granule melting or gelatinization temperature of the starch. After cooking, these products do not retain granular structure but rather agglomerate to form paste-like systems.
U.S. Pat. No. 5,882,713 describes non-separable compositions of starch and water-immiscible materials by soluablization of starch granules in a stream of water and steam under high turbulence conditions. Such products are characterized as: being stable without phase separation on prolonged standing; forming soft gels upon heating; forming dried, non-oily solids upon drying; and easy hydration in water to form stable and homogeneous dispersions. In the process of the '713 patent, starch is fully solubilized and aligned at the interface of oil and water phases to form thick and gel-like films about oil or lipid droplets.
In 1987 Englyst and Cummings at the MRC Dunn Clinical Nutrition Center in Cambridge, UK, proposed a classification of starch based on its likely digestive properties in vivo. They also devised in vitro assay methods to mimic the various digestive properties of starch. Three classes of dietary starch were proposed:
(1) Rapidly Digestible Starch (RDS). RDS is likely to be rapidly digested in the human small intestine; examples include freshly cooked rice and potato, and some instant breakfast cereals.
(2) Slowly Digestible Starch (SDS). SDS is likely to be slowly yet completely digested in the small intestine; examples include raw cereal starch and cooked pasta.
(3) Resistant Starch (RS). RS is likely to resist digestion in the small intestine. RS is thus defined as the sum of starch and starch degradation products not likely to be absorbed in the small intestine of healthy individuals. RS can be subdivided into four categories depending on the cause of resistance (Englyst et al Eur. J. Clin. Nutr. 46(suppl 2):S33, 1992; Eerlingen et al Cereal Chem. 70:339, 1993).                RS1. Physically inaccessible starch due to entrapment of granules within a protein matrix or within a plant cell wall, such as in partially milled grain or legumes after cooling.        RS2. Raw starch granules, such as those from potato or green banana, that resist digestion by α-amylase, possibly because those granules lack micropores through their surface.        RS3. Retrograded amylose formed by heat/moisture treatment of starch or starch foods, such as occurs in cooked/cooled potato and corn flake.        RS4. Chemically modified starches, such as acetylated, hydroxypropylated, or cross-linked starches that resist digestion by alpha-amylase. Those modified starches would be detected by the in vitro assay of RS. However, some RS4 may not be fermented in the colon.        
RS1, RS2, RS3 are physically modified forms of starch and become accessible to α-amylase digestion upon solubilization in sodium hydroxide or dimethyl sulfoxide. RS4 that is chemically substituted remains resistant to α-amylase digestion even if dissolved. RS4 produced by cross-linking would resist dissolution.
U.S. Pat. No. 6,299,907 describes improved resistant starches which are modified so as to be reversibly swellable. These starch products have a number of novel properties including the capability of undergoing multiple cycles of swelling and drying while substantially retaining the individuality of the starch granules and with the presence of very small amounts of starch solubles. These products also exhibit swelling powers enabling them to absorb water in excess of their own weights. However, when mixed with hot or cold water, these starch products eventually precipitate toward the bottom of the water phase, which may limit wide applicability of the products in water/oil emulsion systems.
U.S. Pat. No. 5,151,264 and Publication U.S. 2002/0168408 describe starch-based vectors used for the transport of biologically active molecules. These vectors include a cross-linked starch core with a first layer or ring of lipid material bonded to the core and a second layer or outer shell of amphiphilic compounds bonded to the lipid layer. However, these references do not describe the use of a preswelled or expanded starch core.
U.S. Pat. No. 5,858,993 describes starch-metal complexes useful for accelerating the healing of topical wounds or as a hair growth stimulant. The complexes described in the '993 patent are prepared by using native starch granules which are first pasted and then reacted with relatively high concentrations of copper(II) or tin(II) salts. There is no teaching in this patent regarding metal substitution of cross-linked starches or expanded or preswelled starches.